Introduce OutcomeGenerator

[martintmk]

The issue or feature being addressed

Contributes to #1093 and #1094

Details on the issue fix or feature implementation

This PR introduces the base class for all generator events that generate the output value from the outcome. It also introduces the RetryDelayGenerator that is able to generate delays based on user-provided callbacks.

Usage:

new RetryStrategyOptions()
{
    RetryDelayGenerator =
        new RetryDelayGenerator()
            // generates the delay for all int-based results
            .SetGenerator<int>((outcome, args) => TimeSpan.FromSeconds(args.Attempt))
};

In the next PR I'll introduce the OutcomeEvent and the core infra for callbacks should be done.

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov-commenter]

Codecov Report

Merging #1095 (1b35c02) into main (305650e) will increase coverage by 0.24%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main    #1095      +/-   ##
==========================================
+ Coverage   75.72%   75.97%   +0.24%     
==========================================
  Files         178      182       +4     
  Lines        4363     4407      +44     
  Branches      808      813       +5     
==========================================
+ Hits         3304     3348      +44     
  Misses        854      854              
  Partials      205      205              

Flag	Coverage Δ
linux	75.97% <100.00%> (+0.24%)	⬆️
macos	75.97% <100.00%> (+0.24%)	⬆️
windows	75.97% <100.00%> (+0.24%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Polly.Core/Retry/RetryDelayArguments.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryDelayGenerator.cs	100.00% <100.00%> (ø)
src/Polly.Core/Retry/RetryStrategyOptions.cs	100.00% <100.00%> (ø)
...rc/Polly.Core/Strategy/OutcomeGenerator.Handler.cs	100.00% <100.00%> (ø)
src/Polly.Core/Strategy/OutcomeGenerator.cs	100.00% <100.00%> (ø)

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